1. Field of the Invention
This invention relates to turbocharger control and more particularly relates to achieving predictive control of a turbocharger charge pressure.
2. Description of the Related Art
Turbochargers have been a common feature on combustion engines to enhance performance. Continuing demand for power density and torque combined with emissions regulations have required continuous innovation in turbocharger hardware and control systems. Hardware related to turbocharger control like wastegates, bypass valves, variable geometry, and adjustable vanes have been known in the art for some time. While improvements in turbocharger hardware are ongoing, turbocharger control systems are largely challenged by limits imposed by present computer processor speeds and the responsiveness of turbocharger speed sensors, which contribute to a lack of control precision during transient events in real time applications.
Present turbocharger control systems are primarily reactive systems that track only one or two system parameters to determine turbocharger speeds. Reactive turbocharger control systems using limited system parameter input may fail to predict the onset of surge and choke events that may result in suboptimal performance and stress to turbocharger components. For example, a turbocharger turbine wheel experiencing a surge event may cease rotating for periods of time, which diminishes the protective oil film covering on the turboshaft, resulting in deterioration of the turbocharger load capacity. Often, conditions resulting from surge and choke events in the turbocharger means that turbocharger control systems are very conservatively calibrated to their operating environments and thereby their performance and efficiency may be adversely affected. Present turbocharger control systems must choose between aggressively responding turbocharger control that may overshoot target speeds and target pressures in highly transient events, or less responsive turbocharger control that may not experience overshoot, but that experiences reduced transient performance. A turbocharger experiencing numerous surge and/or choke events not only adversely affects the performance of an engine but may reduce the service life and reliability of the turbocharger.
Some present turbocharger systems include more than one turbocharger stage. These systems allow greater charge densities and compression of intake air. Further, they allow the inclusion of a smaller, high pressure turbocharger to improve turbocharger response, and the inclusion of a larger, low pressure turbocharger to improve turbocharger flow capacity. However, small turbochargers are even more susceptible to surge and choke events during transience due to the low mass of the turbine wheel.